1. Field of the invention
This invention relates generally to the analysis and process for indicating the amplitude distribution of random-amplitude pulse signals, such as those generated by a scintillation detector / photomultiplier.
2. Description of the prior art
Such devices are commonly used in many technical areas where measurements involve nuclear particles and radiation detection, one among others being e.g. the well logging techniques, wherein a tool is lowered in a well to carry out physical measurements. Of the many well logging instruments and techniques developed over the years to determine the hydrocarbon content and productivity of earth formations, the spectroscopy tool, by which energy spectra of the constituents of formation matrices and fluids are generated, has been found to provide information of particular value in formation analysis. Typically, the energy spectra are obtained by detecting either natural gamma rays or gamma rays resulting from the interaction between formation nuclei and high energy neutrons irradiating the formation, and converting each detected gamma ray into an electrical pulse whose amplitude is a measure of the gamma ray energy. These pulses are then sorted according to height in a pulse height analyzer to develop energy spectra characteristic of the constituents of the earth formations near the tool.
The book "Radiation Detection and Measurement" by Glenn F. Knoll (1979) depicts a typical pulse height analyzer, especially pages 720-725. The analyzer is disposed at the output of a scintillator detector linked to a photomultiplier, and is adapted to carry out an analog-to-digital conversion. A Prior art analyzer is usually comprised of two main components, i.e. an analog/digital converter (hereafter referred to as ADC) and a memory. It also comprises, upstream of the ADC, an input gate which prevents pulses from reaching the ADC when the latter is busy, and another linear gate controlled by a single channel analyzer adapted to determine whether the amplitude is above a given threshold, and thus being representative of a pulse. Electronic filters, for amplifying and signal shaping purposes, are also provided at the input of the pulse height analyzer. Since, basically, an ADC can only process one pulse at a time, simultaneous pulses, or successive pulses very close one to the other in time, cannot be processed, and thus detected; in other words, these pulses are lost, thus altering the reliability of the measurement. Accordingly, the faster the analyzer works, the more pulses are detected. However, the known analyzers comprise analogic circuitry, which implies (i) relative low speed processing (ii) relative high costs and (iii) drift with time or temperature. The only attempt made until now for obviating the consequences of the low processing speed rely on discrimination circuits, which distinguish single pulses from stacked pulses, also called "pile-up" pulses; however, said circuits increase the complexity of the analyzer without giving a satisfactory answer to this problem.
Furthermore, since such analyzers, depending upon the field of use, are located either downhole, in aircraft or in satellites, where room is limited and environmental conditions are severe, the pulse height analyzer should be compact and made of high reliability electronic components. Nevertheless, known analyzers are made of numerous components, thus involving relative bulk, as well as difficult and costly adjustments due to functional dispersion.
Moreover, a method has already been proposed wherein the analogic pulse signals are digitized and stored during a time window and wherein the digital data accumulated in the memory are then processed by a computer. However, this known delayed process is time consuming and is not consistent with the requirements of the real time spectrum analysis technique.